Abstract: Presently, the water-cooled wall of boiler with high parameters uses mostly the tube with internal threads, which affects the reflection of ultrasonic at inner surface, so the manual ultrasonic test for welds becomes a problem. The paper introduced the experimentation and research on the effect of internal threads on ultrasonic test. Based on current research, the system of ultrasonic imaging test with small diameter tube was designed, which was able to modify the effect of internal thread in real time, and to carry out the whole-position scanning for welds and exactly judge the inner quality of workpiece(welds).